Fullerenes are broadly defined as the third form of the element carbon after diamond and graphite. Fullerenes are molecular solids that consist of fused six-membered and five-membered rings. Two general types of fullerenes may be described: Buckyballs and carbon nanotubes. Buckyballs have spherical structures and are typified by C60. Other spherical fullerenes include C70 and higher oligomers. Single walled carbon nanotubes (SWNTs) are elongated members of the fullerene family.
The interior cavity of a fullerene can accommodate an atom, molecule, or particle, depending on the volume circumscribed by the structure of the fullerene, to provide so-called doped fullerenes. Furthermore, fullerenes may be chemically functionalized by reacting the surface under suitable conditions to form either covalent, van der Waals or dipolar interactions with a chemical substituent.
SWNTs have come under intense multidisciplinary study because of their unique physical and chemical properties and their possible applications. The electronic characteristics of SWNT can be described as metallic or semiconducting; such characteristics deriving from the helicity and diameter of the SWNT. More importantly, it has been shown that these electronic properties are sensitive to the environment surrounding the SWNT. For example, it is well known that the presence of certain molecules, such as O2 or NH3, may alter the overall conductivity of SWNTs through the donation or acceptance of electrons. Such properties make SWNT ideal for nanoscale sensing materials. Nanotube field effect transistors (FETs), for example, have already been demonstrated as gas sensors. It is thought that selectivity in nanotube sensors can be achieved through the placement of specific functional groups on the nanotube surface; such groups having the requisite ability to selectively bind specific target molecules. Working against this goal is the fact that functionalization changes the electronic properties from that of a semiconductor or conductor to that of an insulator. Moreover, chemical functionalization of SWNT is not as of yet regiospecific. A further major obstacle to such efforts has been diversity of tube diameters, chiral angles, and aggregation states of the tubes. Aggregation is particularly problematic because highly polarizable, smooth sided SWNTs readily form bundles or ropes with van der Waals binding energy of ca. 500 eV per micrometer of tube contact. This bundling perturbs the electronic structure of the tubes and precludes the separation of SWNTs by size or type.
SWNT-based composites can provide excellent electronic and/or mechanical properties upon incorporation into a suitable matrix. Carbon nanotubes are excellent candidates for the fabrication of robust composites, and conducting polymers, due to their fascinating electronic and mechanical properties. Unfortunately, two issues must be overcome prior to development of large-scale applications. First, the SWNTs must be stable within a desired matrix. Second, the aggregation of SWNTs into ropes and bundles requires high loading that is uneconomic and represents a waste of materials.
The first of these issues requires that the SWNTs be protected from subsequent processing, e.g., oxidation. In addition, the formation of a stable tube/matrix interface is critical for composite applications. Surface treatments are required to ensure efficient tube-matrix interactions. Unfortunately, these treatments can result in the degradation of the tubes. The second of these issues requires that individual SWNTs (rather than bundles) be employed to maximize the impact of the SWNTs at the lowest possible loading.
It has been shown that individual SWNTs may be obtained encased in a cylindrical micelle, by ultrasonically agitating an aqueous dispersion of raw SWNTs in the presence of a suitable surfactant (O'Connell et al., 2002). Upon drying the micellular solution, however, bundles re-form. SWNTs have been encased in a wide range of organic materials. It would be desirable to fabricate individually coated SWNTs where the coating is retained in solution and the solid state. Of particular interest are dielectric materials such as silica, which may also be compatible with composite matrix materials. Silica is an example of an inorganic oxide.
Coating of SiO2 on multiwalled carbon nanotubes (MWNTs) has been reported (Seeger et al., 2001). However, these coatings required a sol-gel type of reaction and extremely long reaction times on the order of 150 hours. Coatings have also been reported on SWNTs, but these require isolation of the tubes on a surface prior to reaction. It would be advantageous if there was a method by which individual fullerenes and individual SWNT could be coated under near ambient temperatures with reaction times on the order of a few hours, without the need for isolation on a surface prior to coating.
The classical sol-gel process for generating thin films of an oxide, such as silica, on substrates can be divided into three steps. First, preparation of a stable dispersion of colloidal oxide particles in a liquid, “sol formation”. Second, aggregation of the particles to encompass the liquid, “gel formation”, and deposition of the resulting gel on the surface of the substrate. Third, removal of the solvent by drying and/or heating (Vossen, et al., 2000).
In contrast, the liquid phase deposition, “LPD”, of silica from saturated fluorosilicic acid solutions involves the reaction of water with silica precursors that are solvated at the molecular level to generate silica gels that deposit onto the surface of the substrate (Yeh, et al., 1994). Whereas film growth in the sol-gel method is largely dependent on the size of the initial colloidal particles and its influence on their aggregation, growth in the LPD method is more controlled since it continues layer by layer as more molecules react on the surface of the substrate. The important step in LPD is to provide an active site for growth to occur on a surface.
The semiconductor industry has targeted the development of the interlayer and intrametal dielectric for the next several generations of higher density, faster computer chips, as specified by the milestones set out in the International Technology Roadmap for Semiconductors (the ITRS.) There is still no acceptable material or process that produces films with the desired values of low dielectric constant (k value) concurrently with optimum electro- and thermo-mechanical properties. Current processes are based either on sol-gel methods for film deposition and growth, or on low temperature chemical vapor deposition (CVD) of carbon or fluorine-containing silicon dioxide films. The k values achieved by these processes are in the range from ˜2.7 to greater than 3, still well above the maximum value of 2 required by the industry in order to meet the chip performance milestones identified in the ITRS.
Silicon dioxide (SiO2) forms the basis of planar silicon chip technology. Insulator coatings for electronic and photonic devices layers are most frequently formed by thermal oxidation of silicon (Si) in the temperature range 900 to 1200° C. SiO2 is also deposited by chemical vapor deposition (CVD) techniques at lower temperatures (200 to 900° C.) on various substrates. The growth of insulator films at low temperatures is very attractive for most device applications due to reduced capital cost, high output and freedom from technological constraints associated with the growth of dielectric thin films using conventional high-temperature growth/deposition techniques. Deposition of SiO2 insulator layers from solution is previously known using organo-metallic solutions. In this procedure, the insulator layer is applied onto the substrate either by dipping the substrate into the solution or by spinning the substrate after a small amount of the solution is applied onto the surface. In both cases the substrate is then placed in an oven to drive off the solvent.
Attempts to produce porous silicon dioxide have failed to produce films with isolated voids and uniform void size, resulting in poor process reproducibility and film quality. Such processes also require the use of heat to evaporate a solvent or other component from the film to create the voids, something not required by the present invention.
CVD (chemical vapor deposition) requires the pyrolysis or photolysis of volatile compounds to create chemical fragments that are deposited on the surface of a substrate. The temperature of substrate is sufficiently high to allow mobility of fragments on the growth surface. These fragments travel around the surface until they find thermodynamically stable sites to which they attach. In this way the quality of CVD films is usually high. Thus, CVD uses surface growth. If gas phase growth occurs, uniform films are not produced. Instead, nanoparticles can form, from which films form after agglomeration. The resulting film requires further thermal processing in order to become uniform. Disadvantages with CVD include the high temperatures required and the use of volatile compounds or low pressures. Each of these adds to the environmental load of the process. Sol-gel is low temperature method. Precursor compounds are dissolved in solution and reacted with additional reagents (usually water or an acid) to give a gel. If a film or coating is required, then the gel must be spin-coated onto the substrate. Since most sol-gels consist of nanoparticles or clusters with a significant organic content, additional thermal or chemical treatments are required to form a true inorganic material.